Microporous membranes and methods for producing and using such membranes

ABSTRACT

The invention relates to microporous polymeric membranes suitable for use as battery separator film. The invention also relates to a method for producing such a membrane, batteries containing such membranes as battery separators, methods for making such batteries, and methods for using such batteries.

CROSS-REFERENCE OF RELATED APPLICATIONS

This application claims priority from U.S. Prov. App. Ser. No.61/115,410, filed 17 Nov. 2008, U.S. Prov. App. Ser. No. 61/115,405,filed 17 Nov. 2008, EP09151320.0 filed 26 Jan. 2009, and EP09151318.4filed 26 Jan. 2009, the contents of each of which are incorporated byreference in their entirety.

FIELD OF THE INVENTION

The invention relates to multi-layer microporous polymeric membranessuitable for use as battery separator film. The invention also relatesto a method for producing such a membrane, batteries containing suchmembranes as battery separators, methods for making such batteries, andmethods for using such batteries.

BACKGROUND OF THE INVENTION

Microporous membranes can be used as battery separators in, e.g.,primary and secondary lithium batteries, lithium polymer batteries,nickel-hydrogen batteries, nickel-cadmium batteries, nickel-zincbatteries, silver-zinc secondary batteries, etc. When microporouspolyolefin membranes are used for battery separators, particularlylithium ion battery separators, the membranes' characteristicssignificantly affect the properties, productivity and performance of thebatteries. Accordingly, it is desirable for the microporous membrane tohave resistance to thermal shrinkage, particularly at elevatedtemperature. Resistance to thermal shrinkage (or “heat shrinkage”) canimprove the battery's protection against internal short circuiting thatmight otherwise occur as the separator shrinks away from the edges ofthe battery's electrodes at elevated temperature.

European Patent Application Publication No. EP 1 905 586 (published Feb.2, 2008) discloses multi-layer polymeric membranes useful as batteryseparator film. One of the membranes exemplified has a transversedirection heat shrinkage of 2% at 105° C.

Japanese patent document JP2000198866 (published Jul. 18, 2000)discloses multi-layer battery separator films having heat shrinkagevalues of 10%. The membrane comprises layers containing alpha-olefin-COcopolymers and an inorganic species (cross-linked silicone powders).

PCT publication WO2007-049568 (published May 3, 2007) also disclosesmulti-layer battery separator films having a machine directionheat-shrinkage value of 4% and a transverse direction heat shrinkagevalue of 3%. The films of this reference comprise a core layercontaining heat-resistant polymers or an inorganic filler.

U.S. Patent Publication 2007/0218271 discloses monolayer microporousfilms having machine and transverse direction heat shrinkage values of4% or less. The films of this reference are produced from high densitypolyethylene having a weight-average molecular weight of 2×10⁵ to 4×10⁵,containing not more than 5 wt. % of molecules with a molecular weight of1×10⁴ or less and not more than 5 wt. % of molecules having a molecularweight of 1×10⁶ or more.

Japanese Patent Application Laid Open No. JP2001-192487 disclosesmonolayer microporous membranes having transverse direction heatshrinkage values as low as 1.8%, but at a relatively low permeability(Gurley value of 684 seconds). Similarly, Japanese Patent ApplicationLaid Open No. JP2001-172420 discloses monolayer microporous membraneshaving transverse direction heat shrinkage values as low as 1.1%, but ata Gurley value above 800.

While improvements have been made, there is still a need for batteryseparator film having increased resistance to heat shrinkage.

SUMMARY OF THE INVENTION

In one embodiment, the invention relates to a microporous membranecomprising polypropylene having an Mw>0.9×10⁶, the membrane having a130° C. heat shrinkage ≦8.0% in at least one planar direction and anormalized air permeability ≦4.0×10² seconds/100 cm³.

In another embodiment, the invention relates to a method for producing amicroporous membrane, comprising,

-   -   (a) stretching a multi-layer layer extrudate in at least one of        MD or TD, the extrudate comprising at least first and second        layers, the first layer comprising a first polyolefin and at        least a first diluent, and the second layer comprising a second        polyolefin and at least a second diluent, the second polyolefin        comprising polypropylene in an amount in the range of from 1.0        wt. % to 40.0 wt. % based on the weight of the second        polyolefin, the polypropylene having an Mw>0.9×10⁶ and a        ΔHm≧100.0 J/g;    -   (b) removing at least a portion of the first and second diluents        from stretched extrudate to produce a dried membrane having a        first length along MD and a first width along TD;    -   (c) stretching the membrane in MD from the first length to a        second length larger than the first length by a first        magnification factor in the range of from about 1.1 to about 1.5        and stretching the membrane in TD from the first width to a        second width that is larger than the first width by a second        magnification factor in the range of from about 1.1 to about        1.3; and then    -   (d) reducing the second width to a third width, the third width        being in the range of from the first width to about 1.1 times        larger than the first width.

In yet another embodiment, the invention relates to a battery comprisingan anode, a cathode, an electrolyte, and at least one battery separatorlocated between the anode and the cathode, the battery separatorcomprising the microporous membrane of any of the preceding embodiments.The battery can be, e.g., a lithium ion primary or secondary battery.The battery can be used as a power source, e.g., as a power source for apower tool such as a battery-operated saw or drill.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a cross-sectioned, perspective view showing one example ofcylindrical type lithium ion secondary battery comprising an electrodeassembly of the present invention.

FIG. 2 is a cross-sectioned view showing the battery in FIG. 1.

FIG. 3 is an enlarged cross-sectioned view showing a portion A in FIG.2.

DETAILED DESCRIPTION OF THE INVENTION

One battery failure mode involves the high temperature softening ofmembranes used as battery separator film and a loss of dimensionalstability especially near the edges of the membrane. Should the width ofthe membrane decrease at a temperature above the membrane's shutdowntemperature (generally much higher than 105° C.), the close spacingbetween anode, cathode, and separator can lead to an internal shortcircuit in the battery. This is particularly the case in prismatic andcylindrical batteries, where even a small change in membrane width canresult in anode-cathode contact at or near the battery's edges.

The invention relates to the discovery of microporous membranes having arelatively high meltdown temperature (≧170.0° C.), relatively high pinpuncture strength (≧2.0×10³ Mn, e.g., ≧3.0×10³ Mn), and improved heatshrinkage properties, i.e., better dimensional stability at elevatedtemperature. The improvement in heat shrinkage properties is observednot only at relatively low temperatures (e.g., below about 110° C.,which is within the operating temperature range of conventional lithiumion batteries), but also at relatively high temperatures (e.g., above125° C., or above 135° C., e.g., above the shutdown temperature ofconventional battery separator film for lithium ion batteries).

Since a battery separator film might not be softened sufficiently at105° C. to exhibit poor heat shrinkage, the film's heat shrinkageperformance at 105° C. is not always a reliable indicator of thepotential for internal battery short circuiting. In contrast, the film'smaximum heat shrinkage in the molten state is measured at a temperaturethat is above the membrane's shutdown temperature, and thus can be abetter indicator for this type of internal short circuiting. Maximumheat shrinkage in the molten state is generally not predictable solelyfrom the membrane's heat shrinkage performance at 105° C.

Structure and Composition of the Multi-Layer Microporous Membrane

In an embodiment, the microporous membrane comprises first and secondlayers. The first layer comprises a first layer material, and the secondlayer comprises an independently selected second layer material. Thefirst and second layer materials can be, e.g., independently selectedpolyolefins. For example, the membrane has a planar top layer whenviewed from above on an axis approximately perpendicular to planar axesalong the length and width of the membrane, and a planar bottom layerthat is parallel or approximately parallel to the top layer. In anotherembodiment, the multi-layer microporous membrane comprises three or morelayers, e.g., a membrane having first and third layers and a secondlayer located between the first and third layers. While the third layercan comprise an independently selected third layer material, this is notrequired. When the multi-layer microporous membrane has three or morelayers, at least one layer comprises the first microporous layermaterial and at least one layer comprises the second microporous layermaterial. In an embodiment, the first and third layers are produced from(and generally comprise) substantially the same polymer or mixture ofpolymers (e.g., both are produced from the first layer material).

In an embodiment, the multi-layer, microporous membrane comprises threelayers, wherein the first and third layers (also called the “surface” or“skin” layers) comprise outer layers of the membrane and the secondlayer is an intermediate layer (or “core” layer) located between thefirst and third layers. In a related embodiment, the multi-layer,microporous membrane can comprise additional layers, i.e., in additionto the two skin layers and the core layer. For example, the membrane cancontain additional core layers between the first and third layers. Themembrane can be a coated membrane, i.e., it can have one or moreadditional layers on or applied to the first and third layers.Generally, the second layer of the membrane has a thickness of 5% to 15%of the membrane's total thickness; and the first and third layers of themembrane have the same thickness, the thickness of the first and thirdlayer each being in the range of 42.5% to 47.5% of the membrane's totalthickness.

Optionally, the core layer is in planar contact with one or more of theskin layers in a stacked arrangement such as A/B/A with face-to-facestacking of the layers. The membrane can be referred to as a “polyolefinmembrane” when the membrane contains polyolefin. While the membrane cancontain polyolefin only, this is not required, and it is within thescope of the invention for the polyolefin membrane to contain polyolefinand materials that are not polyolefin. Optionally, the polyolefins canbe produced, e.g., in a process using a chromium catalyst, aZiegler-Natta catalyst, or a single-site polymerization catalyst. Forthe purpose of this description and the appended claims, the term“polymer” means a composition including a plurality of macromolecules,the macromolecules containing recurring units derived from one or moremonomers. The macromolecules can have different size, moleculararchitecture, atomic content, etc. The term “polymer” includesmacromolecules such as copolymer, terpolymer, etc., and encompassesindividual polymer components and/or reactor blends. The term“polyolefin” means polymer containing recurring units derived fromolefin, e.g., poly-α olefin such as polypropylene and/or polyethylene.“Polypropylene” means polyolefin containing recurring propylene-derivedunits, e.g., polypropylene homopolymer and/or polypropylene copolymerwherein at least 85% (by number) of the recurring units are propyleneunits. “Polyethylene” means polyolefin containing recurringethylene-derived units, e.g., polyethylene homopolymer and/orpolyethylene copolymer wherein at least 85% (by number) of the recurringunits are ethylene units. The first and second layer materials will nowbe described in more detail.

In an embodiment, the first layer material comprises a firstpolyethylene having a weight-average molecular weight (“Mw”)≦1.0×10⁶ anda second polyethylene having an Mw>1.0×10⁶. The third layer materialcomprises a first polyethylene having an Mw≦1.0×10⁶ and a secondpolyethylene having an Mw>1.0×10⁶. The second layer material comprisespolypropylene. It is conventional for microporous membranes used as BSFsto contain polypropylene for achieving high meltdown temperatures, e.g.,≧170.0° C. But adding sufficient polypropylene (e.g., ≧2.0 wt. % basedon the weight of the membrane) to achieve a high meltdown temperaturegenerally decreases high temperature stability (e.g., increases heatshrinkage, especially at elevated temperature) and decreases pinpuncture strength. In an embodiment, the membranes of the inventionovercome this difficulty.

For example, in one embodiment the second layer material comprises afirst polyethylene having an Mw≦1.0×10⁶, polypropylene having anMw>0.9×10⁶, and optionally a second polyethylene having an Mw>1.0×10⁶.Optionally, the first polyethylene of the second and/or third layermaterial is the same first polyethylene as in the first layer material.Optionally, the second polyethylene of the second and/or third layermaterial is the same second polyethylene as in the first layer material.In an embodiment, neither the first nor third layer material containspolypropylene in an amount greater than 0.5 wt. %. In a relatedembodiment, the first and/or third layer material consists essentiallyof polyethylene, e.g., substantially the same polyethylene orcombination of polyethylenes.

In an embodiment, the first layer material comprises from about 90.0 wt.% to about 99.0 wt. %, e.g., 92.5 wt. % to about 97.5 wt. %, of thefirst polyethylene and from about 1.0 wt. % to about 1.0 wt. %, e.g.,from about 2.5 wt. % to about 7.5 wt. %, of the second polyethylene; theweight percents being based on the weight of the first layer material.In an embodiment, the polyethylenes of the third layer material areselected from among substantially the same polyethylenes inapproximately the same concentration ranges as the first layer material.

In an embodiment, the second layer material comprises the firstpolyethylene, the polypropylene, for example ≦40.0 wt. % polypropylene,and optionally the second polyethylene. For example, the second layermaterial can comprise from about 60.0 wt. % to about 95.0 wt. %, of thefirst polyethylene, from about 5.0 wt. % to about 40.0 wt. % of thepolypropylene, and from about 0.0 wt. % to about 10.0 wt. % of thesecond polyethylene, the weight percents being based on the weight ofthe second layer material. In another embodiment, the second layermaterial comprises from about 60.0 wt. % to about 75.0 wt. % of thefirst polyethylene, from about 25.0 wt. % to about 35.0 wt. % of thepolypropylene, and from about 0.5 wt. % to about 5.0 wt. % of the secondpolyethylene, the weight percents being based on the weight of thesecond layer material. In an embodiment, the microporous membranecomprises polypropylene in an amount ≦8.0 wt. % based on the totalweight of the microporous membrane, e.g., in the range of 2.2 wt. % to7.0 wt. %.

Optionally, the microporous membranes contain copolymers, inorganicspecies (such as species containing silicon and/or aluminum atoms),and/or heat-resistant polymers such as those described in PCTPublications WO 2007/132942 and WO 2008/016174. In an embodiment,membrane is substantially free of such materials. Substantially free inthis context means the amount of such materials in the microporousmembrane is ≦1.0 wt. %, based on the total weight of the polymer used toproduce the microporous membrane.

The final microporous membrane generally comprises the polymer used toproduce the extrudate. A small amount of diluent or other speciesintroduced during processing can also be present, generally in amounts≦1.0 wt. % based on the weight of the microporous membrane. A smallamount of polymer molecular weight degradation might occur duringprocessing, but this is acceptable. In an embodiment, the Mw of thepolymers in the membrane decrease by a factor of ≦10%, for example, or≦1.0%, or ≦0.1% of the Mw of the polymers used to produce the membrane.

The polypropylene, the first and second polyethylenes, and the diluentsused to produce the extrudate and the microporous membrane will now bedescribed in more detail. The invention is not limited to theseembodiments, and the following description is not meant to forecloseother embodiments within the broader scope of the invention.

Materials Used to Produce the Microporous Membrane

In an embodiment, the first and third layer materials are produced fromthe first diluent and the first and second polyethylenes; and the secondlayer material is produced from the second diluent, the firstpolyethylene, the polypropylene, and optionally the second polyethylene.Optionally, inorganic species (such as species containing silicon and/oraluminum atoms), and/or heat-resistant polymers such as those describedin PCT Publications WO 2007/132942 and WO 2008/016174 (both of which areincorporated by reference herein in their entirety) can be used toproduce the first, second, and/or third layer materials. In anembodiment, these optional species are not used.

A. The First Polyethylene

In an embodiment, the first polyethylene has an Mw≦1.0×10⁶, e.g., in therange of from about 1.0×10⁵ to about 0.90×10⁶, a molecular weightdistribution (“MWD”) in the range of from about 2.0 to about 50.0, and aterminal unsaturation amount <0.20 per 1.0×10⁴ carbon atoms. (“PE1”).Optionally, the first polyethylene has an Mw in the range of from about4.0×10⁵ to about 6.0×10⁵, and an MWD of from about 3.0 to about 10.0.Optionally, the first polyethylene has an amount of terminalunsaturation ≦0.14 per 1.0×10⁴ carbon atoms, or ≦0.12 per 1.0×10⁴ carbonatoms, e.g., in the range of 0.05 to 0.14 per 1.0×10⁴ carbon atoms(e.g., below the detection limit of the measurement). PE1 can be a highdensity polyethylene (“HDPE”), e.g., SH-800® polyethylene, availablefrom Asahi.

In another embodiment, the first polyethylene has an Mw<1.0×10⁶, e.g.,in the range of from about 2.0×10⁵ to about 0.9×10⁶, an MWD in the rangeof from about 2 to about 50, and a terminal unsaturation amount ≧0.20per 10,000 carbon atoms. (“PE2”). Optionally, the first polyethylene hasan amount of terminal unsaturation ≧0.30 per 1.0×10⁴ carbon atoms, or≧0.50 per 1.0×10⁴ carbon atoms, e.g., in the range of 0.6 to 10.0 per1.0×10⁴ carbon atoms. A non-limiting example of the first polyethyleneis one having an Mw in the range of from about 3.0×10⁵ to about 8.0×10⁵,for example about 7.5×10⁵, and an MWD of from about 4 to about 15. PE2can be HDPE, e.g., Lupolen®, available from Basell. The firstpolyethylene can be a mixture of PE1 and PE2.

The first polyethylene can be, e.g., an ethylene homopolymer or anethylene/α-olefin copolymer containing ≦5.0 mole % of one or morecomonomer such as α-olefin, based on 100% by mole of the copolymer.Optionally, the α-olefin is one or more of propylene, butene-1,pentene-1, hexene-1,4-methylpentene-1, octene-1, vinyl acetate, methylmethacrylate, or styrene. PE1 can be produced, e.g., in a process usinga Ziegler-Natta or single-site polymerization catalyst, but this is notrequired. The amount of terminal unsaturation can be measured inaccordance with the procedures described in PCT Publication WO97/23554,for example. PE2 can be produced using a chromium-containing catalyst,for example.

The Mw and MWD (MWD defined as Mw/Mn where Mn is the number averagemolecular weight) of the first polyethylene are determined using a HighTemperature Size Exclusion Chromatograph, or “SEC”, (GPC PL 220, PolymerLaboratories), equipped with a differential refractive index detector(DRI). Three PLgel Mixed-B columns (available from Polymer Laboratories)are used. The nominal flow rate is 0.5 cm³/min, and the nominalinjection volume was 300 μL. Transfer lines, columns, and the DRIdetector were contained in an oven maintained at 145° C. The measurementis made in accordance with the procedure disclosed in “Macromolecules,Vol. 34, No. 19, pp. 6812-6820 (2001)”.

The GPC solvent used is filtered Aldrich reagent grade1,2,4-Trichlorobenzene (TCB) containing approximately 1000 ppm ofbutylated hydroxy toluene (BHT). The TCB is degassed with an onlinedegasser prior to introduction into the SEC. Polymer solutions areprepared by placing dry polymer in a glass container, adding the desiredamount of above TCB solvent, then heating the mixture at 160° C. withcontinuous agitation for about 2 hours. The concentration of UHMWPEsolution was 0.25 to 0.75 mg/ml. Sample solution are filtered off-linebefore injecting to GPC with a 2 μm filter using a model SP260 SamplePrep Station (available from Polymer Laboratories).

The separation efficiency of the column set is calibrated with acalibration curve generated using seventeen individual polystyrenestandards ranging in Mp (“Mp” being defined as the peak in Mw) fromabout 580 to about 10,000,000, which is used to generate the calibrationcurve. The polystyrene standards are obtained from Polymer Laboratories(Amherst, Mass.). A calibration curve (logMp vs. retention volume) isgenerated by recording the retention volume at the peak in the DRIsignal for each PS standard, and fitting this data set to a 2nd-orderpolynomial. Samples are analyzed using IGOR Pro, available from WaveMetrics, Inc.

B. The Second Polyethylene

In an embodiment, the second polyethylene has an Mw>1.0×10⁶, e.g., inthe range of from about 1.0×10⁶ to about 5.0×10⁶ and an MWD of fromabout 1.2 to about 50.0. A non-limiting example of the secondpolyethylene is one having an Mw of from about 1.0×10⁶ to about 3.0×10⁶,for example about 2.0×10⁶, and an MWD of from about 2.0 to about 20.0,preferably about 4.0 to 15.0. The second polyethylene can be, e.g., anethylene homopolymer or an ethylene/α-olefin copolymer containing ≦5.0mole % of one or more comonomers such as α-olefin, based on 100% by moleof the copolymer. The comonomer can be, for example, one or more of,propylene, butene-1, pentene-1, hexene-1,4-methylpentene-1, octene-1,vinyl acetate, methyl methacrylate, or styrene. Such copolymer can beproduced using a Ziegler-Natta or a single-site catalyst, though this isnot required. The second polyethylene can be ultra-high molecular weightpolyethylene (“UHMWPE”), e.g., 240-m® polyethylene, available fromMitsui.

The Mw, Mn, and MWD of the second polyethylene are determined the sameway as for the first polyethylene.

C. The Polypropylene

In an embodiment, the polypropylene has an Mw>0.9×10⁶, for example fromabout 1.0×10⁶ to about 2.0×10⁶, such as from about 1.1×10⁶ to about1.5×10⁶. Optionally, the polypropylene has an MWD≦100, e.g., from about1.1 to about 50.0, or about 2.0 to about 6.0; and/or a heat of fusion(“ΔHm”)≧100.0 J/g, e.g., 110 J/g to 120 J/g, such as from about 113 J/gto 119 J/g or from 114 J/g to about 116 J/g. The polypropylene can be,for example, one or more of (i) a propylene homopolymer or (ii) acopolymer of propylene and a ≦10.0 mole % comonomer, based on 100% bymole of the entire copolymer. The copolymer can be a random or blockcopolymer. The comonomer can be, for example, one or more of α-olefinssuch as ethylene, butene-1, pentene-1, hexene-1,4-methylpentene-1,octene-1, vinyl acetate, methyl methacrylate, and styrene, etc.; anddiolefins such as butadiene, 1,5-hexadiene, 1,7-octadiene,1,9-decadiene, etc. Optionally, the polypropylene has one or more of thefollowing properties: (i) the tacticity is isotactic; (ii) anelongational viscosity of at least about 50,000 Pa sec at a temperatureof 230° C. and a strain rate of 25 sec⁻¹; (iii) a melting peak Tm(second melt) of at least about 160° C., e.g., greater than about 166°C., or even greater than about 168° C., or even greater than about 170°C. (the melting point can be determined by conventional methods, e.g.,differential scanning calorimetry (DSC)); (iv) a Trouton's ratio of atleast about 15 when measured at a temperature of about 230° C. and astrain rate of 25 sec⁻¹; (v) an elongational viscosity of at least about5.0×10⁴ Pa sec at a temperature of 230° C. and a strain rate of 25sec⁻¹; (vi); an Mw≧1.75×10⁶, or ≧2.0×10⁶, or ≧2.25×10⁶, such as, forexample ≧2.5×10⁶; (vii) a Melt Flow Rate (MFR) at 230° C. and 2.16 kgweight of ≦about 0.01 dg/min (i.e., a value is low enough that the MFRis essentially not measurable; Melt Flow Rate is determined inaccordance with conventional methods, such as ASTM D 1238-95 ConditionL); (viii) exhibits stereo defects of ≦50.0 per 1.0×10⁴ carbon atoms, or≦40.0, or ≦30.0, or even ≦20.0 per 1.0×10⁴ carbon atoms, e.g., thepolypropylene can have fewer than about 10.0, or fewer than about 5.0stereo defects per 1.0×10⁴ carbon atoms; (ix) a meso pentad fraction ofgreater than about 96 mol % mmmm pentads; and/or (x) an amountextractable species (extractable by contacting the polypropylene withboiling xylene) of ≦0.5 wt. %, or ≦0.2 wt. %, or ≦0.1 wt. % based on theweight of the polypropylene.

The polypropylene's ΔHm, is determined by the methods disclosed in PCTPatent Publication No. WO2007/132942, which is incorporated by referenceherein in its entirety. Tm can be determined from differential scanningcalorimetric (DSC) data obtained using a PerkinElmer Instrument, modelPyris 1 DSC. Samples weighing approximately 5.5-6.5 mg are sealed inaluminum sample pans. The DSC data are recorded by first heating thesample to 200° C. at a rate of 150° C./minute, called first melt (nodata recorded). The sample is kept at 200° C. for 10 minutes before acooling-heating cycle is applied. The sample is then cooled from 200° C.to 25° C. at a rate of 10° C./minute, called “crystallization”, thenkept at 25° C. for 10 minutes, and then heated to 200° C. at a rate of10° C./minute, called (“second melt”). The thermal events in bothcrystallization and second melt are recorded. The melting temperature(T_(m)) is the peak temperature of the second melting curve and thecrystallization temperature (T_(c)) is the peak temperature of thecrystallization peak.

Meso pentad fraction can be determined from ¹³C NMR data obtained at 100MHz at 125° C. on a Varian VXR 400 NMR spectrometer. A 90° C. pulse, anacquisition time of 3.0 seconds, and a pulse delay of 20 seconds areemployed. The spectra are broad band decoupled and acquired withoutgated decoupling. Similar relaxation times and nuclear Overhausereffects are expected for the methyl resonances of polypropylenes, whichare generally the only homopolymer resonances used for quantitativepurposes. A typical number of transients collected is 2500. The sampleis dissolved in tetrachlorethane-d₂ at a concentration of 15% by weight.All spectral frequencies are recorded with respect to an internaltetramethylsilane standard. In the case of polypropylene homopolymer,the methyl resonances are recorded with respect to 21.81 ppm for mmmm,which is close to the reported literature value of 21.855 ppm for aninternal tetramethylsilane standard. The pentad assignments used arewell established.

The amount of extractable species (such as relatively low molecularweight and/or amorphous material, e.g., amorphous polyethylene) isdetermined by solubility in xylene at 135° C., according to thefollowing procedure. Weigh out 2 grams of sample (either in pellet orground pellet form) into 300 ml conical flask. Pour 200 ml of xyleneinto the conical flask with stir bar and secure the flask on a heatingoil bath. Turn on the heating oil bath and allow melting of the polymerby leaving the flask in oil bath at 135° C. for about 15 minutes. Whenmelted, discontinue heating, but continue stirring through the coolingprocess. Allow the dissolved polymer to cool spontaneously overnight.The precipitate is filtered with Teflon filter paper and then driedunder vacuum at 90° C. The quantity of xylene soluble is determined bycalculating the percent by weight of total polymer sample (“A”) lessprecipitate (“B”) at room temperature [soluble content=((A−B)/A)×100].

The Mw and Mn of the polypropylene are determined by the methoddisclosed in PCT Patent Publication No. WO2007/132942, which isincorporated by reference herein in its entirety. Methods of producingthe microporous membrane

In an embodiment, the multi-layer microporous membrane of the inventionis a two-layer membrane. In another embodiment, the multi-layermicroporous membrane has at least three layers. Although the inventionis not limited thereto, the method for producing the microporousmembrane will mainly be described in terms of a three layer membranehaving first and third layers comprising the first layer material and asecond layer comprising the second layer material located between thefirst and third layers.

One method for producing the multi-layer microporous membrane involveslayering, such as by lamination or coextrusion of extrudates ormembranes, e.g., monolayer extrudates or monolayer microporousmembranes. For example, one or more layers comprising the first layermaterial can be coextruded with one or more layers comprising the secondlayer material, e.g., with the layers comprising the first layermaterial located on one or both sides of the layers (or layers)comprising the second layer material.

In an embodiment, the process for producing the membrane involvescooling a multilayer extrudate having a first planar direction (e.g.,the machine direction of extrusion or “MD”) and an orthogonal secondplanar direction (e.g., the direction transverse to MD, called thetransverse direction or “TD”). The extrudate can comprise at leastfirst, second, and third layers, wherein the second layer is locatedbetween the first and third layers. The first and third layers of theextrudate comprise the first layer material and at least a firstdiluent, and the second layer of the extrudate comprises the secondlayer material and at least a second diluent. The first and third layerscan be outer layers of the extrudate, also called skin layers. Thoseskilled in the art will appreciate that the third layer of the extrudatecould be produced from a different layer material, e.g., the third layermaterial, and could have a different thickness than the first layer. Theprocess also involves stretching the cooled extrudate in MD and/or TDand removing at least a portion of the first and second diluents fromstretched extrudate to produce a dried membrane having a first drylength in the in the first planar direction and a first dry width in thesecond planar direction. The process then involves stretching the driedmembrane along MD and optionally TD to form the final membrane. Anembodiment for producing a three-layer membrane will now be described inmore detail. The invention is not limited to these embodiments, and thefollowing description is not meant to foreclose other embodiments withinthe broader scope of the invention.

Combining the First Layer Material and First Diluent

The first layer material is produced by combining the first polyethyleneand optionally second polyethylene e.g., by dry mixing or melt blending.The combined polymers can be combined with one or more diluents to forma mixture of polymer and diluent to produce a first mixture. Thepolymers can be in the form of polymer resins. The diluents can be,e.g., solvents for the polymers of the first layer material. When thediluents are such solvents, the diluent can be called a membrane-formingsolvent and the combined polymer and diluent can be called a polymericsolution, e.g., a polyolefin solution. The first mixture can optionallycontain additives such as one or more antioxidant. In an embodiment, theamount of such additives is ≦1.0 wt. % based on the weight of themixture of polymer and diluent.

The first mixture is then produced, the first mixture comprising a firstdiluent and the first layer material. Optionally, the first diluent is asolvent that is liquid at room temperature, although the diluent can beany species or mixture of species capable of forming a single phase withthe first layer material at the extrusion temperature. While not wishingto be bound by any theory or model, it is believed that the use of aliquid solvent to form the first polyolefin solution makes it possibleto conduct stretching of the extrudate (generally a gel-like sheet) at arelatively high stretching magnification.

Examples of the diluent include at least one of aliphatic or cyclichydrocarbon such as nonane, decane, decalin and paraffin oil, andphthalic acid ester such as dibutyl phthalate and dioctyl phthalate.Paraffin oil with kinetic viscosity of 20-200 cSt at 40° C. can be used.The choice of first diluent, mixing condition, extrusion condition, etc.can be the same as those disclosed in PCT Publication No. WO2008/016174, for example, which is incorporated by reference herein inits entirety.

The amount of first diluent in the combined diluent and first layermaterial in the first polyolefin solution is not critical. In anembodiment, the amount of first diluent is in the range of 20.0 wt. % to9.09 wt. %, e.g., 25.0 wt. % to 50.0 wt. %, based on the combined weightof first diluent and first layer material.

Combining the Second Layer Material and Second Diluent

A second mixture (comprising the second layer material and seconddiluent) can be combined by the same methods used to produce the firstmixture. For example, the polymer comprising the second layer materialcan be combined by melt-blending the first polyethylene, thepolypropylene, and optionally the second polyethylene. The seconddiluent can be selected from among the same diluents as the firstdiluent. And while the second diluent can be (and generally is) selectedindependently of the first diluent, the diluent can be the same as thefirst diluent, and can be used in the same relative concentration as thefirst diluent is used in the first polyolefin solution.

In an embodiment, the method for preparing the second mixture from themethod for preparing the first mixture, in that the mixing temperatureis preferably in a range from the melting point (Tm2) of thepolypropylene to Tm2+90° C.

Extrusion

In an embodiment, the first mixture is conducted from a first extruderto first and third dies and the second mixture is conducted from asecond extruder to a second die. A layered extrudate in sheet form(i.e., a body significantly larger in the planar directions than in thethickness direction) can be extruded (e.g., coextruded) from the first,second, and third dies to produce a multi-layer extrudate having skinlayers comprising the first mixture, and a core layer comprising thesecond mixture.

The choice of die or dies and extrusion conditions can be the same asthose disclosed in PCT Publication No. WO 2008/016174, for example.

Cooling the Multilayer Extrudate

The multilayer extrudate can be exposed to a temperature in the range of15° C. to 25° C. to form a cooled extrudate. Cooling rate is notparticularly critical. For example, the extrudate can be cooled at acooling rate of at least about 30° C./minute until the temperature ofthe extrudate (the cooled temperature) is approximately equal to theextrudate's gelation temperature (or lower). Process conditions forcooling can be the same as those disclosed in PCT Publication No. WO2008/01617, for example. In an embodiment, the cooled extrudate has athickness ≦10 mm, e.g., in the range of 0.1 mm to 1.0 mm, or 0.5 mm to5.0 mm. Generally, the second layer of the cooled extrudate has athickness of 5.0% to 15.0% of the cooled extrudate's total thickness;and the first and third layers of the cooled extrudate havesubstantially the same thickness, the thickness of the first and thirdlayer each being in the range of 42.5% to 47.5% of the cooledextrudate's total thickness.

Stretching the Extrudate

The extrudate is stretched (referred to as “wet” stretching or wetorientation since the diluent is still present in the extrudate) in atleast one direction (e.g., at least one planar direction, such as MD orTD) to produce a stretched extrudate. Optionally, the extrudate isstretched simultaneously in the TD and MD to a magnification factor inthe range of 4 to 6. The magnification factor operates multiplicativelyon film size. For example, a film having an initial width (TD) of 2.0 cmthat is stretched in TD to a magnification factor of 4 fold will have afinal width of 8.0 cm. Suitable stretching methods are described in PCTPublication No. WO 2008/016174, for example. While not required, the MDand TD magnifications can be the same. In an embodiment, the stretchingmagnification is equal to 5 in MD and TD.

While not required, the stretching can be conducted while exposing theextrudate to a temperature in the range of from about the Tcdtemperature Tm. Tcd and Tm are defined as the crystal dispersiontemperature and melting point of the polyethylene having the lowestmelting point among the polyethylenes used to produce the extrudate(i.e., the first and second polyethylene). The crystal dispersiontemperature is determined by measuring the temperature characteristicsof dynamic viscoelasticity according to ASTM D 4065. In an embodimentwhere Tcd is in the range of about 90.0 to 100.0° C., the stretchingtemperature can be from about 90 to 125° C.; preferably form about 100.0to 125.0° C., more preferably from 105.0 to 125.0° C.

In an embodiment, the stretched extrudate undergoes an optional thermaltreatment before diluent removal. In the thermal treatment, thestretched extrudate is exposed to a temperature that is higher (warmer)than the temperature to which the extrudate is exposed duringstretching. The planar dimensions of the stretched extrudate (length inMD and width in TD) can be held constant while the stretched extrudateis exposed to the higher temperature. Since the extrudate containspolymer and diluent, its length and width are referred to as the “wet”length and “wet” width. In an embodiment, the stretched extrudate isexposed to a temperature in the range of 120.0° C. to 125.0° C. for atime in the range of 1.0 second to 1.0×10² seconds while the wet lengthand wet width are held constant, e.g., by using tenter clips to hold thestretched extrudate along its perimeter. In other words, during thethermal treatment, there is no magnification or demagnification (i.e.,no dimensional change) of the stretched extrudate in MD or TD.

In this step and in other steps such as dry orientation and heat settingwhere the sample (e.g., the extrudate, dried extrudate, membrane, etc.)is exposed to an elevated temperature, this exposure can be accomplishedby heating air and then conveying the heated air into proximity with thesample. The temperature of the heated air, which is generally controlledat a set point equal to the desired temperature, is then conductedtoward the sample through a plenum for example. Other methods forexposing the sample to an elevated temperature, including conventionalmethods such as exposing the sample to a heated surface, infra-redheating in an oven, etc. can be used with or instead heated air.

Removal of the First and Second Diluents

In an embodiment, at least a portion of the first and second diluents(e.g., membrane-forming solvents) are removed (or displaced) from thestretched extrudate to form a dried membrane. A displacing (or“washing”) solvent can be used to remove (wash away, or displace) thefirst and second diluents. Process conditions for removing first andsecond diluents can be the same as those disclosed in PCT PublicationNo. WO 2008/016174, for example. The term “dried membrane” refers to anextrudate from which at least a portion of the diluent has been removed.It is not necessary to remove all diluent from the stretched extrudate,although it can be desirable to do so since removing diluent increasesthe porosity of the final membrane.

In an embodiment, at least a portion of any remaining volatile species,such as washing solvent, can be removed from the dried membrane at anytime after diluent removal. Any method capable of removing the washingsolvent can be used, including conventional methods such as heat-drying,wind-drying (moving air), etc. Process conditions for removing volatilespecies such as washing solvent can be the same as those disclosed inPCT Publications No. WO 2008/016174 and WO 2007/132942, for example.

Stretching the Dried Membrane

The dried membrane is stretched (called “dry stretching”) in at leastMD. A dried membrane that has been dry stretched is called an “oriented”membrane. Before dry stretching, the dried membrane has an initial sizein MD (a first dry length) and an initial size in TD (a first drywidth). As used herein, the term “first dry width” refers to the size ofthe dried membrane in the transverse direction prior to the start of dryorientation. The term “first dry length” refers to the size of the driedmembrane in the machine direction prior to the start of dry orientation.Tenter stretching equipment of the kind described in WO 2008/016174 canbe used, for example.

The dried membrane can be stretched in MD from the first dry length to asecond dry length that is larger than the first dry length by a firstmagnification factor (the “MD dry stretching magnification factor”) inthe range of from about 1.1 to about 1.5. When TD dry stretching isused, the dried membrane can be stretched in TD from the first dry widthto a second dry width that is larger than the first dry width by asecond magnification factor (the “TD dry stretching magnificationfactor”). Optionally, the TD dry stretching magnification factor is ≦the MD dry stretching magnification factor. The TD dry stretchingmagnification factor can be in the range of from about 1.1 to about 1.3,such as from about 1.15 to about 1.25. The stretching (also calledre-stretching since the diluent-containing extrudate has already beenstretched) can be sequential or simultaneous in MD and TD. Since TD heatshrinkage generally has a greater effect on battery properties than doesMD heat shrinkage, in one embodiment the first magnification factor is >the second magnification factor.

When TD dry stretching is used, the dry stretching can be simultaneousin MD and TD or sequential. When the dry stretching is sequential,generally MD stretching is conducted first followed by TD stretching.

The dry stretching is generally conducted while exposing the driedmembrane to a temperature ≦Tm, e.g., in the range of from aboutTcd-30.0° C. to Tm. In an embodiment where the membrane is a multi-layermembrane having first and third layers comprising polyethylene and asecond layer comprising polypropylene that is located between the firstand third layers, the stretching temperature is generally conducted withthe membrane exposed to a temperature in the range of from about 70.0°C. to about 135.0° C., for example from about 80.0° C. to about 132.0°C. In one embodiment, the MD stretching is conducted before TDstretching, and

-   -   i) The MD stretching is conducted while the membrane is exposed        to a first temperature in the range of Tcd-30.0° C. to about        Tm-10.0° C., for example 70.0° C. to about 125.0° C., or about        80.0° C. to about 120.0° C. and    -   ii) The TD stretching is conducted while the membrane is exposed        to a second temperature that is higher than the first        temperature but lower than Tm, for example about 70.0° C. to        about 135.0° C., or about 127.0° C. to about 132.0° C., or about        129.0° C. to about 131.0° C.

In an embodiment, the MD dry stretching magnification factor is in therange of from about 1.1 to about 1.5, such as 1.2 to 1.4; and the TD drystretching magnification factor is in the range of from about 1.1 toabout 1.3, such as 1.15 to 1.25, with the MD stretching first, followedby stretching in the TD direction.

The stretching rate is preferably 3.0%/second or more in the stretchingdirection (MD or TD), and the rate can be independently selected for MDand TD stretching. The stretching rate is preferably 5%/second or more,more preferably 10.0%/second or more, e.g., in the range of 5.0%/secondto 25.0%/second. Though not particularly critical, the upper limit ofthe stretching rate is preferably 50.0%/second to prevent rupture of themembrane.

Controlled Reduction of the Membrane's Width

Following the dry stretching, the dried membrane is subjected to acontrolled reduction in width from the second dry width to a thirdwidth, the third dry width being in the range of from the first drywidth to about 1.1 times larger than the first dry width. The widthreduction generally conducted while the membrane is exposed to atemperature ≦Tcd-30.0° C., but less than Tm. For example, during widthreduction the membrane can be exposed to a temperature in the range offrom about 70.0° C. to about 135.0° C., such as from about 127.0° C. toabout 132.0° C., e.g., from about 129.0° C. to about 131.0° C. In anembodiment, the decreasing of the membrane's width is conducted whilethe membrane is exposed to a temperature that is lower than Tm. In anembodiment, the third dry width is in the range of from 1.0 times largerthan the first dry width to about 1.1 times larger than the first drywidth, such as from 1.0 times larger than the first dry width to about1.05 times larger than the first dry width

It is believed that exposing the membrane to a temperature during thecontrolled width reduction that is ≧ the temperature to which themembrane was exposed during the TD stretching leads to greaterresistance to heat shrinkage in the finished membrane.

Optional Heat-Setting

Optionally, the membrane is thermally treated (heat-set) one or moretimes after diluent removal, e.g., after dry stretching, the controlledwidth reduction, or both. It is believed that heat-setting stabilizescrystals and make uniform lamellas in the membrane. In an embodiment,the heat setting is conducted while exposing the membrane to atemperature in the range Tcd to Tm, e.g., a temperature e.g., in therange of from about 100° C. to about 135° C., such as from about 127.0°C. to about 132.0° C., or from about 129.0° C. to about 131.0° C.Generally, the heat setting is conducted for a time sufficient to formuniform lamellas in the membrane, e.g., a time in the range of 1.0 to100.0 seconds. In an embodiment, the heat setting is operated underconventional heat-set “thermal fixation” conditions. The term “thermalfixation” refers to heat-setting carried out while maintaining thelength and width of the membrane substantially constant, e.g., byholding the membrane's perimeter using tenter clips during the heatsetting.

Optionally, an annealing treatment can be conducted after the heat-setstep. The annealing is a heat treatment with no load applied to themembrane, and may be conducted by using, e.g., a heating chamber with abelt conveyer or an air-floating-type heating chamber. The annealing mayalso be conducted continuously after the heat-setting with the tenterslackened. During annealing the membrane can be exposed to a temperaturein the range of Tm or lower, e.g., in the range from about 60° C. toabout Tm-5° C. Annealing is believed to provide the microporous membranewith improved permeability and strength.

Optional heated roller, hot solvent, cross linking, hydrophilizing, andcoating treatments can be conducted if desired, e.g., as described inPCT Publication No. WO 2008/016174.

Optionally, an annealing treatment can be conducted before, during, orafter the heat-setting. The annealing is a heat treatment with no loadapplied to the membrane, and can be conducted by using, e.g., a heatingchamber with a belt conveyer or an air-floating-type heating chamber.The annealing can be conducted continuously, e.g., after theheat-setting with the tenter slackened. The temperature to which themembrane is exposed during annealing, (the “annealing temperature”) canbe, e.g., in a range from about 126.9° C. to 128.9° C. Annealing isbelieved to provide the microporous membrane with improved heatshrinkage and strength.

Optional heated roller, hot solvent, cross linking, hydrophilizing, andcoating treatments can be conducted if desired, e.g., as described inPCT Publication No. WO 2008/016174.

Properties of the Multi-Layer Microporous Membrane

In an embodiment, the membrane is a multi-layer microporous membrane.The membrane's thickness is generally in the range of 3.0 μm or more.For example, the membrane can have a thickness in the range of fromabout 5.0 μm to about 200.0 μm, e.g., from about 10.0 μm to about 50.0μm. The membrane's thickness can be measured, e.g., by a contactthickness meter at 1.0 cm longitudinal intervals over the width of 10.0cm, and then averaged to yield the membrane thickness. Thickness meterssuch as the Litematic available from Mitsutoyo Corporation are suitable.This method is also suitable for measuring thickness variation afterheat compression, as described below. Non-contact thickness measurementmethods are also suitable, e.g. optical thickness measurement methods.

Optionally, the membrane has one or more of the following properties.

A. Porosity

In an embodiment, the membrane has a porosity ≧25.0%, e.g., in the rangeof about 25.0% to about 80.0%, or 30.0% to 60.0%. The membrane'sporosity is measured conventionally by comparing the membrane's actualweight to the weight of an equivalent non-porous membrane of the samecomposition (equivalent in the sense of having the same length, width,and thickness). Porosity is then determined using the formula: Porosity%=100×(w2−w1)/w2, wherein “w1” is the actual weight of the microporousmembrane and “w2” is the weight of an equivalent non-porous having thesame size and thickness.

B1. Normalized Air Permeability

In an embodiment, the membrane's normalized air permeability (Gurleyvalue, normalized to an equivalent membrane thickness of 20.0 μm) is≦4.0×10² seconds/100 cm³/20 μm. Since the air permeability value isnormalized to a film thickness of 20 μm, the air permeability value isexpressed in units of “seconds/100 cm³/20 μm”. In an embodiment, thenormalized air permeability is in the range of 100.0 seconds/100 cm³/20μm to about 400.0 seconds/100 cm³/20 μm, or 150.0 seconds/100 cm³/20 μmto 390.0 seconds/100 cm³/20 μm. Normalized air permeability is measuredaccording to JIS P8117, and the results are normalized to a value at athickness of 20 μm using the equation A=20 μm*(X)/T₁, where X is themeasured air permeability of a membrane having an actual thickness T₁and A is the normalized air permeability at a thickness of 20 μm.

B2. Air Permeability after Heat Compression

In an embodiment, the membrane's air permeability after heat compressionis ≦1.0×10³ seconds/100 cm³, e.g., 500.0 seconds/100 cm³ to 750.0seconds/100 cm³. Air permeability after heat compression is measuredaccording to JIS P8117 after the membrane is subjected to a compressionof 2.2 MPa (22 kgf/cm²) in the thickness direction for five minuteswhile the membrane is exposed to a temperature of 90.0° C.

C. Normalized Pin Puncture Strength

In an embodiment, the membrane has a pin puncture strength ≧2.0×10³mN/20 μm, e.g., in the range of 3.0×10³ mN/20 μm to 5.0×10³ mN/20 μm.Pin puncture strength is defined as the maximum load measured when amicroporous membrane having a thickness of T₁ is pricked with a needleof 1.0 mm in diameter with a spherical end surface (radius R ofcurvature: 0.5 mm) at a speed of 2.0 mm/second. The pin puncturestrength is normalized to a value at a membrane thickness of 20 μm usingthe equation S₂=20 μm*(S₁)/T₁, where S₁ is the measured pin puncturestrength, S₂ is the normalized pin puncture strength, and T₁ is theaverage thickness of the membrane.

D. Tensile Strength

In an embodiment, the membrane has an MD tensile strength ≧9.0×10⁴ kPa,e.g., in the range of 1.0×10⁵ to 1.1×10⁵ kPa, and a TD tensile strength≧5.5×10⁴, in the range of 8.0×10⁴ kPa to 1.0×10⁵ kPa. Tensile strengthis measured in MD and TD according to ASTM D-882A.

E. Tensile Elongation ≧100%

Tensile elongation is measured according to ASTM D-882A. In anembodiment, the membrane's MD and TD tensile elongation are each ≧150%,e.g., in the range of 150% to 350%. In another embodiment, themembrane's MD tensile elongation is in the range of, e.g., 150% to 250%and TD tensile elongation is in the range of, e.g., 150% to 250%.

F. Shutdown Temperature

In an embodiment, the membrane has a shutdown temperature ≦140.0° C.,e.g., in the range of about 132.0° C. to about 138.0° C. The shutdowntemperature of the microporous membrane is measured by athermomechanical analyzer (TMA/SS6000 available from Seiko Instruments,Inc.) as follows: A rectangular sample of 3.0 mm×50.0 mm is cut out ofthe microporous membrane such that the long axis of the sample isaligned with membrane's TD and the short axis is aligned with MD. Thesample is set in the thermomechanical analyzer at a chuck distance of10.0 mm, i.e., the distance from the upper chuck to the lower chuck is10.0 mm. The lower chuck is fixed and a load of 19.6 mN applied to thesample at the upper chuck. The chucks and sample are enclosed in a tubewhich can be heated. Starting at 30.0° C., the temperature inside thetube is elevated at a rate of 5.0° C./minute, and sample length changeunder the 19.6 mN load is measured at intervals of 0.5 seconds andrecorded as temperature is increased. The temperature is increased to200.0° C. “Shutdown temperature” is defined as the temperature of theinflection point observed at approximately the melting point of thepolymer having the lowest melting point among the polymers used toproduce the membrane.

G. Meltdown Temperature

In an embodiment, the membrane's meltdown temperature is ≧170.0° C.,e.g., in the range of 171.0° C. to 200.0° C., or 172.0° C. to 190.0° C.The membrane meltdown temperature is measured by the followingprocedure: A rectangular sample of 3.0 mm×50.0 mm is cut out of themicroporous membrane such that the long axis of the sample is alignedwith the microporous membrane's TD as it is produced in the process andthe short axis is aligned with MD. The sample is set in thethermomechanical analyzer (TMA/SS6000 available from Seiko Instruments,Inc.) at a chuck distance of 10.0 mm, i.e., the distance from the upperchuck to the lower chuck is 10 mm. The lower chuck is fixed and a loadof 19.6 mN applied to the sample at the upper chuck. The chucks andsample are enclosed in a tube which can be heated. Starting at 30.0° C.,the temperature inside the tube is elevated at a rate of 5.0° C./minute,and sample length change under the 19.6 mN load is measured at intervalsof 0.5 second and recorded as temperature is increased. The temperatureis increased to 200.0° C. The meltdown temperature of the sample isdefined as the temperature at which the sample breaks, generally at atemperature in the range of about 145° C. to about 200° C.

In an embodiment, the meltdown temperature is in the range of from170.0° C. to 180.0° C. Since the membrane has a desirably high melt-downtemperature, it is suitable for use as a battery separator inhigh-power, high capacity lithium ion batteries such as those used forpowering electric vehicles and hybrid electric vehicles.

H. Thickness Variation Ratio after Heat Compression

In an embodiment, the membrane's thickness variation ratio after heatcompression is ≦20.0% of the thickness of the membrane before the heatcompression, e.g., in the range of 5.0% to 10.0%. Thickness variationafter heat compression is measured by subjecting the membrane to acompression of 2.2 MPa (22 kgf/cm²) in the thickness direction for fiveminutes while the membrane is exposed to a temperature of 90.0° C. Themembrane's thickness variation ratio is defined as the absolute value of(average thickness after compression−average thickness beforecompression)/(average thickness before compression)×100.

I. Electrolytic Solution Absorption Speed

In an embodiment, the membrane has an electrolytic solution absorptionspeed ≧3.0, e.g., in the range of 3.2 to 5.0. Using a dynamic surfacetension measuring apparatus (DCAT21 with high-precision electronicbalance, available from Eko Instruments Co., ltd.), a microporousmembrane sample is immersed in an electrolytic solution for 600 seconds(electrolyte: 1 mol/L of LiPF₆, solvent: ethylene carbonate/dimethylcarbonate at a volume ratio of 3/7) kept at 18.0° C., to determine anelectrolytic solution absorption speed by the formula of [weight (ingrams) of microporous membrane after immersion/weight (in grams) ofmicroporous membrane before immersion]. The electrolytic solutionabsorption speed is expressed by a relative value, assuming that theelectrolytic solution absorption rate in the microporous membrane ofComparative Example 1 is 1.0. Battery separator film having a relativelyhigh electrolytic solution absorption speed (e.g., ≧3.0) are desirablesince less time is required for the separator to uptake the electrolyteduring battery manufacturing, which in turn increases the rate at whichthe batteries can be produced.

J. Heat Shrinkage at 105° C. in at Least One Planar Direction ≦1.0%

In an embodiment, the membrane has a heat shrinkage at 105° C. in atleast one planar direction (e.g., MD or TD) of ≦1.0% e.g., ≦0.5%, suchas in the range of from 0.1% to 0.25%. The membrane's shrinkage at 105°C. in MD and TD is measured as follows: (i) Measure the size of a testpiece of microporous membrane at ambient temperature in both the MD andTD, (ii) equilibrate the test piece of the microporous membrane at atemperature of 105° C. for 8 hours with no applied load, and then (iii)measure the size of the membrane in both the MD and TD. The heat (or“thermal”) shrinkage in MD and TD can be obtained by dividing the resultof measurement (i) by the result of measurement (ii) and expressing theresulting quotient as a percent.

K. TD Heat Shrinkage at 130° C.

In an embodiment, the membrane has a TD heat shrinkage measured at 130°C.≦8.0%, e.g., 1% to 7.5%. A relatively low heat shrink value, e.g.,≦8.0% can be of particular significance since 130° C. is generallywithin the operating temperature range of a lithium ion secondarybattery during charging and discharging, albeit near the upper(shut-down) end of this range.

The measurement is slightly different from the measurement of heatshrinkage at 105° C., reflecting the fact that the edges of the membraneparallel to the membrane's TD are generally fixed within the battery,with a limited degree of freedom allowed for expansion or contraction(shrinkage) in TD, particularly near the center of the edges parallel tothe membrane's MD. Accordingly, a square sample of microporous filmmeasuring 50 mm along TD and 50 mm along MD is mounted in a frame, withthe edges parallel to TD fixed to the frame (e.g., by tape) leaving aclear aperture of 35 mm in MD and 50 mm in TD. The frame with sampleattached is then heated in thermal equilibrium (e.g., in an oven) at atemperature of 130° C. for thirty minutes, and then cooled. TD heatshrinkage generally causes the edges of the film parallel to MD to bowslightly inward (toward the center of the frame's aperture). Theshrinkage in TD (expressed as a percent) is equal to the length of thesample in TD before heating divided by the narrowest length (within theframe) of the sample in TD after heating times 100 percent.

L. Maximum Heat Shrinkage in Molten State

Maximum shrinkage in the molten state in a planar direction of themembrane is measured by the following procedure:

Using the TMA procedure described for the measurement of meltdowntemperature, the sample length measured in the temperature range of from135° C. to 145° C. are recorded. The membrane shrinks, and the distancebetween the chucks decreases as the membrane shrinks. The maximumshrinkage in the molten state is defined as the sample length betweenthe chucks measured at 23.0° C. (L1 equal to 10 mm) minus the minimumlength measured generally in the range of about 135° C. to about 145.0°C. (equal to L2) divided by L1, i.e., [L1−L2]/L1*100%. When TD maximumshrinkage is measured, the rectangular sample of 3.0 mm×50.0 mm used iscut out of the microporous membrane such that the long axis of thesample is aligned with the transverse direction of the microporousmembrane as it is produced in the process and the short axis is alignedwith the machine direction. When MD maximum shrinkage is measured, therectangular sample of 3.0 mm×50.0 mm used is cut out of the microporousmembrane such that the long axis of the sample is aligned with themachine direction of the microporous membrane as it is produced in theprocess and the short axis is aligned with the transverse direction.

In an embodiment, the membrane's maximum MD heat shrinkage in the moltenstate is ≦25.0% or ≦20.0%, e.g., in the range of 1.0% to 25.0%, or 2.0%to 20.0%. In an embodiment, the membrane's maximum TD heat shrinkage inthe molten state is ≦11.0%, or ≦6.0%, e.g., in the range of 1.0% to10.0%, or 2.0% to 5.5%.

Battery

The microporous membranes of the invention are useful as batteryseparators in e.g., lithium ion primary and secondary batteries. Suchbatteries are described in PCT publication WO 2008/016174 which isincorporated by reference herein in its entirety.

FIG. 1 shows an example of a cylindrical-type lithium ion secondarybattery comprising two battery separators. The microporous membranes ofthe invention are suitable for use as battery separators in this type ofbattery. The battery has a toroidal-type electrode assembly 1 comprisinga first separator 10, a second separator 11, a cathode sheet 13, and ananode sheet 12. The separators' thicknesses are not to scale, and aregreatly magnified for the purpose of illustration. The toroidal-typeelectrode assembly 1 can be wound, e.g., such that the second separator11 is arranged on an outer side of the cathode sheet 13, while the firstseparator 10 is arranged on the inner side of the cathode sheet. In thisexample, the second separator 11 is arranged on inside surface of thetoroidal-type electrode assembly 1, as shown in FIG. 2.

In this example, an anodic active material layer 12 b is formed on bothsides of the current collector 12 a, and a cathodic active materiallayer 13 b is formed on both sides of the current collector 13 a, asshown in FIG. 3. As shown in FIG. 2, an anode lead 20 is attached to anend portion of the anode sheet 12, and a cathode lead 21 is attached toan end portion of the cathode sheet 13. The anode lead 20 is connectedwith battery lid 27, and the cathode lead 21 is connected with thebattery can 23.

While a battery of cylindrical form is illustrated, the invention is notlimited thereto, and the separators of the invention are suitable foruse in e.g., prismatic batteries such as those containing electrodes inthe form of stacked plates of anode(s) 12 and a cathode (3) 13alternately connected in parallel with the separators situated betweenthe stacked anodes and cathodes.

When the battery is assembled, the anode sheet 12, the cathode sheet 13,and the first and second separators 10, 11 are impregnated with theelectrolytic solution, so that the separator 10, 11 (microporousmembranes) are provided with ion permeability. The impregnationtreatment is can be conducted, e.g., by immersing electrode assembly 1in the electrolytic solution at room temperature. A cylindrical typelithium ion secondary battery can be produced by inserting thetoroidal-type electrode assembly 1 (see FIG. 1) into a battery can 23having a insulation plate 22 at the bottom, injecting the electrolyticsolution into the battery can 23, covering the electrode assembly 1 witha insulation plate 22, caulking a battery lid (24, 25, 26, and 27) tothe battery can 23 via a gasket 28. The battery lid functions as ananode terminal.

FIG. 3 (oriented so that the battery lid, i.e., the anode terminal ofFIG. 1, is toward the right) illustrates the advantage of using aseparator having diminished tendency to shrinkage in the transversedirection (with respect to the separator manufacturing process) as thebattery temperature increases. One role of the separator is to preventcontact of the anodic active material layer and the cathodic activematerial layer. In the event of a significant amount of TDheat-shrinkage, the thin edges of the separators 10 and 11 move awayfrom the battery lid (move leftward in FIG. 3), thereby allowing contactbetween the anodic active material layer and the cathodic activematerial layer, resulting in a short circuit. Since the separators canbe quite thin, usually less than 200 μm, the anodic active materiallayer and the cathodic active material layer can be quite close.Consequently, even a small decrease in the amount of separator TDshrinkage at elevated battery temperature can make a significantimprovement in the battery's resistance to internal short circuiting.

The battery is useful as a source or sink of power from one or moreelectrical or electronic components, Such components include passivecomponents such as resistors, capacitors, inductors, including, e.g.,transformers; electromotive devices such as electric motors and electricgenerators, and electronic devices such as diodes, transistors, andintegrated circuits. The components can be connected to the battery inseries and/or parallel electrical circuits to form a battery system. Thecircuits can be connected to the battery directly or indirectly. Forexample, electricity flowing from the battery can be convertedelectrochemically (e.g., by a second battery or fuel cell) and/orelectromechanically (e.g., by an electric motor operating an electricgenerator) before the electricity is dissipated or stored in a one ormore of the components. The battery system can be used as a power sourcefor moving an electric vehicle or hybrid electric vehicle, for example.In one embodiment, the battery is electrically connected to an electricmotor and/or an electric generator for powering an electric vehicle orhybrid electric vehicle.

The present invention will be explained in more detail referring toExamples below without intention of restricting the scope of the presentinvention.

EXAMPLES OF THE INVENTION Example 1 (1) Preparation of First PolyolefinSolution

A first polyolefin composition is prepared by dry-blending (a) 68.6 wt.% of a first polyethylene resin having an Mw of 5.6×10⁵ and an MWD of4.05, (b) 1.4 wt. % second polyethylene resin having an Mw of 1.9×10⁶and an MWD of 5.09, and (c) 30 wt. % polypropylene resin having an Mw of1.1×10⁶, a heat of fusion of 114 J/g and an MWD of 5.0, the percentagesbeing based on the weight of the first polyolefin composition. The firstpolyethylene resin in the composition has a Tm of 135° C. and a Tcd of100° C.

35 wt. % of the resultant first polyolefin composition is charged into afirst strong-blending double-screw extruder having an inner diameter of58 mm and L/D of 42, and 65.0 wt. % of liquid paraffin (50 cst at 40°C.) is supplied to the double-screw extruder via a side feeder toproduce a first polyolefin solution. The weight percents are based onthe weight of the first polyolefin solution. Melt-blending is conductedat 210° C. and 200 rpm.

(2) Preparation of Second Polyolefin Solution

A second polyolefin solution is prepared in the same manner as above bydry-blending (a) 9.0 wt. % of a first polyethylene resin having an Mw of5.6×10⁵ and an MWD of 4.05, and (b) 5.0 wt. % of a second polyethyleneresin having an Mw of 1.9×10⁶ and an MWD of 5.09, the percentages beingbased on the weight of the second polyolefin composition. The firstpolyethylene resin in the composition has a Tm of 135° C. and a Tcd of100° C.

35.0 wt. % of the resultant second polyolefin composition is chargedinto a second strong-blending double-screw extruder having an innerdiameter of 58 mm and L/D of 42, and 65.0 wt. % of liquid paraffin (50cst at 40° C.) is supplied to the double-screw extruder via a sidefeeder to produce the second polyolefin solution. The weight percentsare based on the weight of the second polyolefin solution. Melt-blendingis conducted at 210° C. and 200 rpm.

(3) Production of Membrane

The first and second polyolefin solutions are supplied from theirrespective double-screw extruders to a three-layer-extruding T-die, andextruded therefrom to produce a layered extrudate (also called alaminate) of second polyolefin solution layer/first polyolefin solutionlayer/second polyolefin solution layer at a layer thickness ratio of45/10/45. The extrudate is cooled while passing through cooling rollerscontrolled at 20.0° C., producing an extrudate in the form of athree-layer gel-like sheet. The gel-like sheet is biaxially stretched(simultaneously) in MD and TD while exposed to a temperature of 119° C.(the “biaxial stretching temperature”) to a magnification of 5 fold ineach of MD and TD by a tenter-stretching machine. The stretchedthree-layer gel-like sheet is fixed to an aluminum frame of 20 cm×20 cm,immersed in a bath of methylene chloride controlled at 25° C. for threeminutes to remove the liquid paraffin, and dried by air flow at roomtemperature to produce a dried membrane. The dried membrane is then drystretched. Before dry stretching, the dried membrane has an initial drylength (MD) and an initial dry width (TD). The dried membrane is firstdry-stretched in MD to a magnification of 1.4 fold while exposed to atemperature of 115° C. (the “MD stretching temperature”, resulting in asecond dry length. The membrane's width (TD) remains approximately equalto the initial dry width during the MD dry stretching. The driedmembrane is then dry-stretched in TD to a magnification of 1.2 fold(resulting in a second dry width) while exposed to a temperature of 130°C. (the “TD stretching temperature”). The membrane's length (MD) remainsapproximately equal to the second dry length during the TD drystretching. Following TD dry-stretching, the membrane is subjected to acontrolled reduction in width (TD) from the second dry width to a finalmagnification of 1.0 fold, the final magnification being based on theinitial width of the membrane at the start of dry stretching, whileexposed to a temperature of at 130.0° C. (the “width reductiontemperature”). In other words, width reduction is carried out until themembrane's final width is substantially the same as the membrane'sinitial dry width. The membrane's length (MD) remains approximatelyequal to the second dry length during the width reduction. The membrane,which remains fixed to the batch-stretching machine, is then heat-setwhile exposed to a temperature of 130.0° C. (the “heat set temperature”)for 10 minutes to produce the final multi-layer microporous membrane.

Example 2

Example 1 is repeated except the first polyolefin composition comprises70 wt. % first polyethylene resin, no added second polyethylene resin,and 30 wt. % polypropylene resin; the biaxial stretching temperature is118.5° C.; and the MD dry stretching is carried out to a magnificationof 1.25 at an MD stretching temperature of 120.0° C.

Comparative Example 1

Example 1 is repeated except the first polyolefin composition comprises80 wt. % of the first polyethylene resin, 20.0 wt. % of the secondpolyethylene resin; there is no second polyolefin solution; the firstpolyolefin solution comprises 20.0 wt. % of the first polyolefincomposition and 80.0 wt. % of liquid paraffin; the extrudate is not alayered extrudate, i.e., it has a single layer produced from the firstpolyolefin solution; the biaxial stretching temperature is 115.0° C.;there is no dry stretching in MD or TD; there is no width reductionstep; and the heat set temperature is 127.0° C.

Comparative Example 2

Example 2 is repeated except the biaxial stretching temperature is117.0° C.; there is no MD dry stretching; the TD dry stretching iscarried out to a magnification of 1.4 fold at a TD stretchingtemperature of 126.0° C.; there is no width reduction step; and the heatsetting temperature is 126.0° C.

Comparative Example 3

Comparative Example 2 is repeated except there is no MD dry stretching;the TD width reduction is carried out to a magnification of 1.2 foldbased on the initial dry width before the start of dry stretching; andTD stretching temperature, the width reduction temperature, and the heatset temperature are all 126.0° C.

Comparative Example 4

Comparative Example 2 is repeated except the polypropylene resin has anMw of 1.56×10⁶, a heat of fusion of 78.4 J/g, and an MWD of 3.2.

Comparative Example 5

Example 1 is repeated except first polyolefin composition comprises (a)49.0 wt. % of the first polyethylene resin, (b) 1.0 wt. % of the secondpolyethylene, and (c) 50.0 wt. % of the polypropylene resin; the secondpolyolefin resin comprises (a) 70.0 wt. % of the first polyethyleneresin and (b) 30.0 wt. % of the second polyethylene resin; the firstpolyolefin solution comprises 25.0 wt. % of the first polyolefincomposition and 75.0 wt. % of liquid paraffin; the second polyolefinsolution comprises 28.5 wt. % of the second polyolefin composition and71.5 wt. % of the liquid paraffin; the first and second polyolefinsolutions are extruded to produce a layered extrudate of secondpolyolefin solution/first polyolefin solution/second polyolefin solutionat a layer thickness ratio of 10/80/10; and the TD stretchingtemperature, the width reduction temperature, and the heat settemperature are all 128.0° C.

Comparative Example 6

Example 2 is repeated except the first polyolefin composition of thefirst polyolefin composition comprises 30.0 wt. % of the firstpolyethylene resin and 70.0 wt. % of the polypropylene resin; thebiaxial stretching temperature is 117.5° C.; the MD dry stretchingmagnification is 1.3 fold and the MD stretching temperature is 80° C.;the controlled width reduction is carried out to magnification of 1.1fold, and the based on the size of the membrane at the start ofdry-stretching; and TD stretching temperature, the width reductiontemperature, and the heat set temperature are all 130.0° C.

The properties of the multi-layer microporous membranes of the examplesand comparative examples are shown in Table 1.

TABLE 1 Ex Ex Comp Comp Comp Comp Comp Comp PROPERTIES 1 2 Ex 1 Ex 2 Ex3 Ex 4 Ex 5 Ex 6 Thickness μm 20.0 26.9 20.3 18.4 19.0 19.8 20.0 20.1Air Perm. 185 384 480 280 300 410 280 287 Porosity % 39.6 33.0 40.5 50.349.0 43.8 48.0 47.5 Punct. Strength 3430 3675 4900 4214 4116 4998 41164214 Tensile Strength 102900 107800 142100 117600 117600 137200 112700100450 MD/TD (kPa) 82320 96040 117600 118580 117600 142100 98000 97020Tensile Elongation 170 210 140 150 150 150 190 160 MD/TD (%) 160 220 240145 140 140 180 150 Heat Shrinkage 105° C. MD/TD 7.5 2.0 6.0 7.1 7.1 7.57.5 3.5 0.2 0 5.5 9.9 7.0 10.2 2.4 3.0 Heat Shrinkage 7.1 5.4 34.7 31.227.3 35.1 10.2 9.3 130° C., TD Electrolytic Solution 3.3 3.3 1.0 2.3 2.63.9 3.1 3.1 Absorption Speed Thick. Var. Aft. Heat Comp. % 7.0 8.0 18.08.0 8.0 17.0 7.0 9.0 Air Perm. Aft. Heat Comp. 660 714 1049 780 815 1105580 640 Shutdown Temp. ° C. 136 135 135 135 135 135 136 136 MeltdownTemp. ° C. 172 172 148 179 178 160 175 178 Max. Shrinkage in MoltenState 19.5 9.3 38.0 32.8 28.6 34.5 25.0 12.5 (%) MD/TD 5.0 0.0 32.0 15.810.0 24.2 7.5 4.0

It is noted from Table 1 that the microporous membrane of the presentinvention exhibits well-balanced important properties, including a TDheat shrinkage at 105° C. of 0.5% or less, with good balance of otherthermal and mechanical properties.

On the other hand, the microporous membrane products of the ComparativeExamples exhibit a poorer heat shrinkage at 105° C., and a generallyhigher air permeability (Comparative Example 1), higher air permeabilityafter heat compression, lower meltdown temperature (ComparativeExample 1) and higher thermal mechanical analysis (TMA) maximum TDshrinkage in the molten state at about 140° C. Comparative examples 5and 6 have relatively high TD heat shrinkage at 105° C., it is believed,because those membranes have core layers containing more than 40 wt. %of the polypropylene.

The multi-layer microporous membrane of the present invention withwell-balanced properties and use of such multi-layer microporousmembrane as a battery separator provides batteries having excellentsafety, heat resistance, retention properties and productivity.

All patents, test procedures, and other documents cited herein,including priority documents, are fully incorporated by reference to theextent such disclosure is not inconsistent and for all jurisdictions inwhich such incorporation is permitted.

While the illustrative forms disclosed herein have been described withparticularity, it will be understood that various other modificationswill be apparent to and can be readily made by those skilled in the artwithout departing from the spirit and scope of the disclosure.Accordingly, it is not intended that the scope of the claims appendedhereto be limited to the examples and descriptions set forth herein butrather that the claims be construed as encompassing all the features ofpatentable novelty which reside herein, including all features whichwould be treated as equivalents thereof by those skilled in the art towhich this disclosure pertains.

When numerical lower limits and numerical upper limits are listedherein, ranges from any lower limit to any upper limit are contemplated.

1. A microporous membrane comprising polypropylene having an Mw>0.9×10⁶,the membrane having a 130° C. heat shrinkage ≦8.0% in at least oneplanar direction and a normalized air permeability ≦4.0×10² second/100cm³/20 μm.
 2. The membrane of claim 1, wherein the membrane's TD 105° C.heat shrinkage is 0.5% or less.
 3. The membrane of claim 1, wherein themembrane's TD 105° C. heat shrinkage is 0.25% or less.
 4. The membraneof claim 1, wherein the membrane has an MD maximum shrinkage in themolten state ≦25.0% and a TD maximum shrinkage in the molten state≦11.0%.
 5. The membrane of claim 1, wherein the membrane comprises firstand third layers and a second layer located between the first and thirdlayers, and wherein (a) the first layer comprises from 1.0 wt. % to 10.0wt. %, based on the weight of the first layer, of polyethylene having anMw>1.0×10⁶; (b) the third layer comprises from 1.0 wt. % to 10.0 wt. %,based on the weight of the third layer, of polyethylene having anMw>1.0×10⁶; and (c) the second layer comprises ≦40 wt. % of thepolypropylene, based on the weight of the second layer.
 6. The membraneof claim 5, wherein the first layer further comprises polyethylenehaving an Mw≦1.0×10⁶ in an amount in the range of 90.0 wt. % to 99.0 wt.%, based on the weight of the first layer; the third layer furthercomprises polyethylene having an Mw≦1.0×10⁶ in an amount in the range of90.0 wt. % to 99.0 wt. %, based on the weight of the third layer; andthe second layer further comprises polyethylene.
 7. The membrane ofclaim 5, wherein the second layer comprises 5.0 wt. % to 40.0 wt. % ofthe polypropylene, 0 wt. % to 10.0 wt. % of polyethylene having anMw>1.0×10⁶, and 60.0 wt. % to 95.0 wt % of polyethylene having anMw≦1.0×10⁶, the weight percents being based on the weight of the secondlayer.
 8. The membrane of claim 5, wherein (a) the second layer is inplanar contact with the first layer and the third layer; (b) themembrane's total thickness is in the range of 3.0 μm to 200.0 μm; (c)the first and third layers comprise substantially the samepolyethylenes, the amounts of the polyethylenes in the first layer beingthe same as the amounts of the polyethylenes in the third layer; (d) thesecond layer has a thickness of 5.0% to 15.0% of the membrane's totalthickness; and (e) the first and third layers have substantially thesame thickness, the thickness of the first and third layers each beingin the range of 42.5% to 47.5% of the membrane's total thickness.
 9. Themembrane of claim 1, wherein the membrane has one or more of (1) anormalized permeability in the range of 150.0 seconds/100 cm³/20 um to390.0 seconds/100 cm³/20 μm, (2) porosity ≧25%, (3) a normalized pinpuncture strength ≧2.0×10³ mN/20 μm, (4) an MD tensile strength ≧9.0×10⁴kPa, (5) an TD tensile strength ≧7.5×10⁴ kPa (6) MD tensile elongation≧50%, (7) TD tensile elongation ≧100%, (8) a meltdown temperature≧170.0° C., (9) a shutdown temperature ≦140.0° C., (10) a thicknessvariation ratio after heat compression ≦20.0%, and (11) an airpermeability after heat compression ≦1.0×10³ sec/100 cm³.
 10. Themembrane of claim 1, wherein the polypropylene's ΔHm is in the range offrom 113 J/g to 119 J/g.
 11. A method for producing a microporousmembrane, comprising, (a) stretching a multi-layer layer extrudate in atleast one of MD or TD, the extrudate comprising at least first andsecond layers, the first layer comprising a first polyolefin and atleast a first diluent, and the second layer comprising a secondpolyolefin and at least a second diluent, the second polyolefincomprising polypropylene in an amount in the range of from 1.0 wt. % to40.0 wt. % based on the weight of the second polyolefin, thepolypropylene having an Mw>0.9×10⁶ and a ΔHm≧100.0 J/g; (b) removing atleast a portion of the first and second diluents from stretchedextrudate to produce a dried membrane having a first length along MD anda first width along TD; (c) stretching the membrane in MD from the firstlength to a second length larger than the first length by a firstmagnification factor in the range of from about 1.1 to about 1.5 andstretching the membrane in TD from the first width to a second widththat is larger than the first width by a second magnification factor inthe range of from about 1.1 to about 1.3; and then (d) reducing thesecond width to a third width, the third width being in the range offrom the first width to about 1.1 times larger than the first width. 12.The method of claim 11, further comprising removing at least a portionof any volatile species from the extrudate after step (b).
 13. Themethod of claim 11, wherein (i) the first polyolefin comprises a firstpolyethylene in an amount in the range of from 90.0 wt. % to 99.0 wt. %and a second polyethylene in an amount in the range of from about 1.0wt. % to 10.0 wt. %, the weight percents being based on the weight ofthe first polyolefin, the first polyethylene having an Mw≦1.0×10⁶ andthe second polyethylene having a weight average molecular weight>1.0×10⁶; (ii) the second polyolefin comprises the polypropylene in anamount in the range of 5 wt. % to 40 wt. %, and further comprises (i) afirst polyethylene having an Mw≦1.0×10⁶ in an amount in the range of60.0 wt. % to 90.0 wt. % and (ii) a second polyethylene having anMw>1.0×10⁶ in an amount in the range of from 0.0 wt. % to 10.0 wt. %,the weight percents being based on the weight of the second polyolefin;(ii) the first diluent is present in the first layer of the extrudate inan amount in the range of from about 25.0 wt. % to about 99.0 wt. %based on the weight of the combined weight of the first polyolefin andthe first diluent; and (iv) the second diluent is present in the secondlayer of the extrudate in an amount in the range of from about 25.0 wt.% to about 99.0 wt. % based on the combined weight of second polyolefinand second diluent.
 14. The method of claim 11, wherein the extrudatefurther comprises a third layer comprising a third polyolefin, the thirdpolyolefin comprising a first polyethylene in an amount in the range offrom 90.0 wt. % to 99.0 wt. % and a second polyethylene in an amount inthe range of from about 1.0 wt. % to 10.0 wt. %, the weight percentsbeing based on the weight of the third polyolefin, the firstpolyethylene having an Mw≦1.0×10⁶ and the second polyethylene having anMw>1.0×10⁶.
 15. The method of claim 11, wherein (i) the extrudate is athree-layer extrudate; (ii) the second layer is located between thefirst and third layers and is in planar contact with the first and thirdlayers; (iii) the first polyolefin and the second polyolefin are thesame polyolefin; (iv) the second layer has a thickness of 5.0% to 15.0%of the extrudate's total thickness; and (v) the first and third layershave the same thickness, the thickness of the first and third layerseach being in the range of 42.5% to 47.5% of the extrudate's totalthickness.
 16. The method of claim 11, wherein the first, second, andthird diluents are independently selected from one or more of nonane,decane, decalin, and liquid paraffin.
 17. The method of claim 11,wherein the stretching of step (a) is conducted by simultaneouslystretching the extrudate in MD and TD.
 18. The method of claim 17,wherein during step (c) the MD stretching is conducted before the TDstretching, wherein the TD stretching is conducted to a secondmagnification factor in the range of from 1.15 to 1.25, wherein thefirst magnification factor is > the second magnification factor, andwherein (i) the MD stretching is conducted while the membrane is exposedto a first temperature in the range of Tcd-30.0° C. to about Tm-10.0° C.and (ii) the TD stretching is conducted while the membrane is exposed toa second temperature that is higher than the first temperature but lowerthan Tm; and wherein the reducing of step (d) is conducted while themembrane is exposed to a temperature ≧ the second temperature.
 19. Themethod of claim 18, wherein the third width is in the range of 1.0 to1.05 times the first width.
 20. The method of claim 18, wherein thefirst magnification factor is in the range of 1.1 to 1.4.
 21. A batterycomprising an anode, a cathode, an electrolyte, and a multi-layermicroporous membrane comprising polypropylene having an Mw>0.9×10⁶,wherein the membrane has a 130° C. heat shrinkage ≦8.0% in at least oneplanar direction and a normalized air permeability ≦4.0×10² second/100cm³/20 μm, and wherein the multi-layer microporous membrane separates atleast the anode from the cathode.
 22. The battery of claim 21, whereinthe electrolyte contains lithium ions and the battery is a secondarybattery.
 23. The battery of claim 22, further comprising one or moreresistive and/or reactive components electrically, electrochemically,and/or electromechanically connected to the battery to form a batterysystem, wherein the battery is a source or sink of power to thecomponent(s).
 24. The battery system of claim 23, wherein at least onecomponent comprises means for moving an electric vehicle or hybridelectric vehicle.
 25. The battery system of claim 23, wherein the meanscomprise an electric motor and/or an electric motor, and the battery iselectrically connected to the motor.